Nanotechnology has been heralded as the next major technological leap, in that it is prophesied to yield a variety of substantial advantages in terms of material characteristics, including electronic, optical and structural characteristics. Nanostructured materials as thin films and coatings possess unique properties due to both size and interface effects. Nanostructured materials are generally a broad class of materials, with microstructures modulated in zero to three dimensions on length scales typically less than about 500 nm, for example, less than about 200 nm, e.g., less than about 100 nm. Nanostructured materials find many applications in areas such as electronics, mass spectrometry, catalysis, protection, data storage, optics, and sensors. Nanostructured films and coatings have many advantages over conventional thin films including high surface area, increased hydrophobicity, increased adhesion, and other similar properties.
One particularly interesting use of nanostructured films and coatings is in mass spectrometry applications. Generally speaking, in mass spectrometry, a substance is bombarded with an electron beam having sufficient energy to fragment the molecule. The positive fragments which are produced (cations and radical cations) are accelerated in a vacuum through a magnetic field and are sorted on the basis of mass-to-charge (m/z) ratio in a mass analyzer. Since the bulk of the ions produced in the mass spectrometer carry a unit positive charge, the value m/z is equivalent to the molecular weight of the fragment. Modern advances in mass spectrometry often address problems regarding the handling of liquid or solid samples. As ions are actually analyzed in the vacuum of the mass spectrometer, arguably the most important reaction is the one that converts analytes of interest into gas-phase ions. Historically, the most commonly used ionization processes (for example, electron ionization) occur in two discrete steps: a sample which is adsorbed on a surface of a substrate is first volatilized and then ionized.
The past two decades have seen the development of new ionization techniques for the analysis of non-volatile and thermally labile compounds: Electrospray ionization (ESI) and matrix-assisted desorption/ionization (MALDI). ESI allows for large, non-volatile molecules to be analyzed directly from the liquid phase. Rather than using an electron beam to ionize a sample as with ESI, MALDI ionizes a sample by pulsed laser irradiation of the sample. The sample is co-crystallized with a solid matrix that can absorb the wavelength of light emitted by the laser. Usually the sample and matrix are mixed on a substrate and inserted into the mass spectrometer instrument, and after irradiation the gas-phase ions that are formed are directed toward the mass analyzer. The broad success of matrix-assisted laser desorption/ionization (MALDI) is related to the ability of the matrix to incorporate and transfer energy to the sample. Barber, et al., Nature 293, 270-275 (1981); Karas, et al., Anal. Chem. 60, 2299-2301 (1988); Macfarlane, et al., Science 191, 920-925 (1976); Hillenkamp, et al., Anal. Chem. 63, A1193-A1202 (1991)).
However, one of the drawbacks of MALDI is the presence of the matrix, which facilitates ionization, but also causes a large degree of chemical noise to be observed at m/z ratios below about 700 Daltons (e.g., for low molecular weight samples). As a result, samples with low molecular weights are usually difficult to analyze with MALDI. Recent variations of MALDI have involved direct desorption/ionization without a matrix and have potential for enabling the analyses of low molecular weight compounds. In particular, the desorption/ionization on porous silicon (DIOS) and silicon continuous or columnar thin films has been used as an alternative to MALDI, see, e.g., Siuzdak et al. U.S. Pat. No. 6,288,390; Fonash et al. U.S. Patent Application No. 20020048531 filed Dec. 19, 2000; Thomas, J. J., Shen, Z., Crowell, J. E., Finn, M. G. & Siuzdak, G., “Desporption/ionization on silicon (DIOS): a diverse mass spectrometry platform for protein characterization,” Proc. Natl Acad. 98:4932-4937 (2001); Shen, Z., et al., “Porous silicon as a versatile platform for laser desorption/ionization mass spectrometry,” Anal Chem. 73:612-619 (2001); Cuiffi, et al., “Desorption-ionization mass spectrometry using deposited nanostructured silicon films,” Anal, Chem. 73:1292-1295 (2001); and, Kruse, et al., “Experimental factors controlling analyte ion generation in laser desorption/ionization mass spectrometry on porous silicon,” Anal. Chem. 73:3639-3645 (2001). These methods typically use porous silicon or etched silicon columnar structures to trap analytes deposited on the surface, and laser irradiation to vaporize and ionize them. Most of these demonstrated applications to date have been based on the porous silicon material produced by electrochemically etching a wafer or deposited film of silicon.
Silicon nanowires have been the subject of extensive research in electronics, photonics, optoelectronics, sensing, and other novel device applications. See, e.g., Cui, et al., “Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species,” Science 293;1289-1292 (2001); Cui, et al., “Functional nanoscale electronic devices assembled using silicon nanowire building blocks,” Science 291:851-853 (2001); Huang, et al., “Integrated optoelectronics assembled from semiconductor nanowires,” Abstracts of Papers of the American Chemical Society 224:U308 (2002); Zhou, et al., “Silicon nanowires as chemical sensors,” Chem. Phys. Lett. 369:220-224 (2003); Duan, et al., “Single-nanowire electrically driven lasers,” Nature 421:241-245 (2003); Hahm, et al., “Direct ultrasensitive electrical detection of DNA and DNA sequence variations using nanowire nanosensors,” Nano Lett. 4:51-54 (2004). Silicon nanowires appear to be an ideal platform for surface-based mass spectrometry. In contrast to porous silicon, silicon nanowires are catalyzed and grown on the surface of a substrate and their physical dimensions, composition, density, and position can be precisely controlled at the nanoscale level, thus offering even greater potential for designing mass spectrometry active surfaces. See, e.g., U.S. Ser. No. 60/468,390 filed May 6, 2003, U.S. Ser. No. 60/468,606 filed May 5, 2003, and U.S. Ser. No. 10/792,402 filed Mar. 2, 2004, all three entitled “Nanofiber Surfaces for Use in Enhanced Surface Area Applications”, the entire contents of which are incorporated by reference herein. However, the use of silicon nanowires as substrate surfaces may pose some challenges in terms of manufacturing such surfaces reproducibly for large-scale commercial production.
It would be beneficial to have a direct laser desorption/ionization technique that eliminates the need for matrix compounds, is reliable and relatively inexpensive to implement, and can be used in biomolecular and other analyses with standard MALDI (and other) mass spectrometer instruments. The present invention provides unique nanostructured thin film surfaces to generate high surface area substrates for matrix-free MALDI and other applications as well. By eliminating background peaks of interfering matrix compounds, good analyses of both low and high-molecular weight compounds such as small molecules, proteins, peptides, oligonucleotides, drugs, pesticides, carbohydrates, fatty acids and the like can be produced more quickly and reliably.